Many different methods and apparatus have been described for reducing the amount of impurities in silicon, including, e.g., zone melting, the silane gas distillation, gas injection, acid leaching, slagging and directional solidification. However boron, phosphorous, titanium, iron and some other elements can only be eliminated with currently known processes to the required purity with great difficulty and/or expensive processing steps.
Currently silicon is typically purified by a process that involves reduction and/or thermal decomposition of an exceptionally pure vaporizable compound of silicon such as trichlorosilane. This process is very costly and capital-intensive way of producing silicon, that has a higher purity than is required for some application such as solar cells.